Hot-pressing process



Nov. 30, 1954 F. wEYERHAr-:USER 2,695,426

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United States Patent Gil 2,695,426 Patented Nov. 30, 1954 ice HOT-PRESSING PROCESS Frederick Weyerhaeuser, St. Paul, Minn., assignor to Rock Island Millwork Company, Rock Island, Ill., a corporation of Illinois Application December 1, 1951, Serial N o. 259,377

6 Claims. (Cl. 18-5S) mosetting phenol-formaldehyde resin to form panels.A

The ideal objectives are to produce panels of uniform thickness, uniform density, uniform surface texture and uniform interior. In part the attainment of these objectives depends upon formation of a layer of the mixture to be molded, such that each unit area of the layer contains the same quantity of the same mixture. This is best accomplished by proper preparation of a loose granular mixture and by laying the latter at a uniform density and uniform thickness, whereby only compression and neating of the layer are required to integrate it to panel orm.

Among the known methods of pressing there are two commonly used in the art. One involves placing the layer between hot platens and closing the latter slowly under constant pressure until a predetermined thickness is attained in a time period su'icient to set the binder. This is herein referred to as follow-up pressing because as the layer is compressed and heated the platens follow the material. By this method the characteristics of the material predetermine the thickness of the panel for any predetermined platen pressure.

A second known method is to close the platens on the layer to a predetermined spacing by use of mechanical stops onto which the platens are closed during the pressing. This is referred to hereinafter as pressing to stops. In this method the characteristics of the material predetermine the density of the panel for each thickness predetermined by the stops.

There are practical disadvantages of these two methods as applied to mixtures of comminuted wood and thermosetting binder. In the follow-up pressing there are particles in constant motion until nal thickness is achieved. The surface layers set before nal thickness is reached and hence this constant motion of the particles can interfere with proper bonding. Also the surface particles are `dried and embrittled, and the embrittled particle does not spread out to fill voids; it may split; and it acts as a solid to localize within the layer the pressure from the platens. Voids are thus produced. To some extent these diculties are minimized by wetting with water the surface particles, while the interior ones are dry. This plasticizes the surface particles, delays the drying and embrittlement, and delays the attainment of thermosetting temperatures. method gives no control of thickness, and in production runs, there are over-thick and under-thick panels.

In the method of pressing to stops many of the foregoing disadvantages are overcome, but other practical disadvantages are introduced. With granular material, especially a dry mixture predominating in wood sawdust, a low-density load in a large press has particles blown olf the face of the load by the rush of air escaping from over the load and from the edges of the load by the escape of air from within the mixture, as the platens close.` These particles land on the stops tive their full effectiveness. In general, `the vicinity of such a press during production operation is contaminated with settled and ying particles of material from unpressed loads, and also from pressed panels on which In general, this follow-up pressure .and thus negaselvage fragments are unavoidable. These conditions make it difficult to keep the stops clean.

In addition, the provision of stops takes away some of the platen area which could otherwise be used for panels. Stops, therefore, result in lowered production by inefcient use of platen area.

Another disadvantage of stops is the necessity to change them to vary the thickness of the panel. In a multi-opening press, which is the conventional commercial press, this difficulty is multiplied.

Still another disadvantage of stops is the extensive area of stop needed for the high pressures used in making adequately dense panels. Under certain conditions the back pressure from the material being pressed reduces to zero or to very low amounts during the cure, in which case the stops must absorb the entire pressure. This requires an area for the stops suicient to absorb the full applied pressure and a platen thickness sufficient to prevent deflection of the platen. This situation calls for more expensive equipment and higher cost of operation.

The present invention involves a method of pressing which overcomes all the above mentioned disadvantages of pressing to stops, which retains all the advantages of pressing to stops, and which offers new advantages not available in either of the two methods discussed above.

It is the general object of the invention to press initially to final thickness while substantially all of the mass is mobile, and to exert all the pressure on the material being pressed.

It is a further object of the invention, after initially pressing to final thickness to release the applied pressure as the back pressure from the material diminishes.

Various other and ancillary objects and advantages of the invention will become apparent from the following description and explanation of the invention, as set forth in connection with the accompanying drawing, showing a graph relating pressure and thickness in carrying out the invention.

The process is carried out by loading a tray or carrier sheet uniformly over each unit area with the same amount of a uniform mixture. Preferably, by suitable means and controls, this is accomplished by loading `a flat bottomed tray to a uniform depth with the material at a uniform density. The density is preferably that of the free-flowing mixture, as it settles by gravity in a layer which may vary in depth over a wide range, for example 1.5 to 2.5 inches for a quarter inch thick panel of about 60 lbs. per cu. ft. However the range may vary to include 8 inches and even l/z inch, depending on many factors. The density of such a layer may vary widely to include a range from 8 to 14 lbs. per cu. ft. depending upon the particle sizes of the comminuted wood, moisture content, and other factors. v

The initial load may be compressed to any density greater than said free-ilowing density before the heat is applied under molding pressure, in order to effect a reduction in the amount of air trapped in the load and to minimize the elfect of air escaping from between the platens on the load surface. In hot-molding loads from conventional mill waste, panels free from voids may be made by compressing to a density of at least 5() lbs. per cut. ft. Higher densities may be attained, the range from 50 to 65 lbs. per cu. ft. being readily achieved by conventional equipment. Densities as high as the maximum compressibility of wood are possible, which varies slightly with the kind of wood and content of resin. 'In general this is near 80 pounds per cu. ft.

The mixture may have a variable moisture content. It may be uniformly dry, and sawdust having up to 35% `by weight of moisture is considered dry at ordinary room temperatures. The loaded mixture may be non-uniform in moisture content, having a dry interior and wet facial layers as above referred to.

For the preferred practice of the present pressing method, such wet faces are not needed. However, when the pressing cycle is unduly long and the platen temperature unduly high, as may be the case where very thick panels are molded, or if the wood particles are relatively coarse, wet faces are advantageous to delay the quick rise in temperature of the facial layers.

The initialmixture should have a moisture content of at least 6% by weight. Otherwise the sawdust particles will be too dry and brittle, and in the initial and a suitably quick compression will fracture in preference to deforining and inter-meshing one with another to. eliminate voi s.

Where thermosetting resin powder isused as thebinder, such as a hot-flowing normally solid powder of a condensation product of phenol and formaldehyde, the mixture may have from 4 to 25 parts of such binder per 100 parts of wood (on the oven-dry basis). For converting millwork sawdust intoA synthetic lumber of a density of about 55 pounds per cu. ft., ay small amount of such resin is suitable, for' example, 6 to 8 parts by weight per 100 parts of wood (oven-dry basis).

Inl operation of comercial presses, it is not practicable to secure substantially instantaneous positioning of the platens. The pressure on the platens and' their relative movement are eifected hydraulically, and hence movement is slow or fast according to the operation'` or limitations of the hydraulic system'.

In a commercial press with reference to which this invention is described, it takes about one to one and onehalf minutes to move the platens from full opening (at which the load is inserted) to full closure to final thickness of the panel. The` pressing time dependsupon thickness'of panel, temperature, and materials, thetinal density of the panel having little inuence in the method of this invention. In short, the. present invention calls for compressing the load to nal thickness as quickly as thev mechanical limitations of the press permit, considering. also the limitations imposed by air over and in the ll being so compressed. The time should be such that substantially all the material of the ll is unset and4 mobilev during the compression.

In` Fig. l there is shown aplot of a pressing in which the timeV from starting closure ofthe platens to release ofthe formed panel is 5.25 minutes, as plotted on the-horizontal axis 10. The left hand vertical axis` 11 is graduated in. pressure of pounds per square inch (p. s. i.) exerted by the platens. The right hand vertical axis 12 is graduated in inches for the thickness of the material from load to panel. By recording pressure and thickness during the pressing, and plotting theseV against time, the relationship is plotted as pressure curve 14 and thickness curve 15.

In the operationy the load ina tray to a depth of 11/2 yinches at a density of lOl/2 pounds per cu. ft. isplaced on a lower platen at a. temperature of 335 F. which is moved up against an upper platen at a like temperature. At 15 seconds, pressure and thickness are first measured, the material having beeny already slightly compressed. The pressure is shown at point 16-as being 500 p. s. i. and the thickness .500 inches. The aim is to press to a final thickness, so'the platens are allowed to move to the corresponding predetermined separation at which movement is ar- `rested. This may be done by positioning an' electric switch so as to close at the exact moment. The switch may effect a signal to-an operator, or it may automatically operate tohold the platens at that position.

Fig., l shows that as the platens close, the pressure rises and falls, but. the thickness diminishes. In this particular press, the limit. of pressure which may be applied is 800 p. s. i.,.whic h limit is reflected in the. plotted curve 14. At 3.0 seconds point- 17 shows themaximum pressure at 800 p.. s. i. and point 181 shows the thickness to be .350 inch. At. 45 seconds, point 1,9 shows continuation of the maximum pressure to be 800 p. s. i. and point 20 shows the thickness to be .280 inch. The dotted pressure line 21 indicates extension of maximum pressurewhich could prevail until something happensin the lill to lessen the back pressure.

Experience with the operation has taught that as soon as the: resin begins to set the back pressure rapidly and almost momentarily diminishes. As a ,consequence there is achieved a sudden drop in back pressure. To maintain the desired nal spacing the reduction iin back pressure requires corresponding reduction in the operation pressure which tends. to move the platens. The operating pressure and the back pressure must balance each other to maintain a fixed spacing. Accordingly, the tendencies of the platens to move reflect the changes in the back pressure andV hence the progress of the setting.

Where the pressure is controlled either manually or automatically by an operator on controlled materials in production runs, an operator acquires a feel or has an indication by which he knows the conditions in the press.

When the expected feel or indication is not encountered the operator is instantly aware of something amiss.

Fig. l further shows thatat 60 seconds the platens have already attained their predetermined panel-thickness separation. However, it is apparent that the critical change occurred between 45 and 60 seconds. Up to the critical point all the particles have been rearranging themselves and fitting together, under the applied force of the closing platens. Less and less pressure is required to hold the platens. Point 24 shows the nal thickness of .260 inch reached at a time between 45 and 60 seconds. Point 25 shows the pressure at 400 p. s. i. at 60 seconds, having dropped from 800 at the critical point somewhere between 45 and 60 seconds. In holding the platens at the xed location, the pressure in the second minute of the operation reduces to 300 p. s. i.. as shown at 26. At the third, fourth and fifth minutes the pressure is constant at 280 p. s. i. shown at 27, 28 and 29. As binder sets the particles are fixed and. back pressure lessens. This is part of the feeL At the` end ofy the fourth. minute, the platens: are opened; the pressure drops to zero as shown at 30, and the panel expands to a thickness. of .270 inch.

In principle, the controlV of the platens is such as to maintain the` platen opening exactly at the` predetermined final thickness of the panel (allowing forexpansion. on release). Practically, this control may be effected. by visual indicating means observed by an operator who controls the hydraulic pressure which moves the platens relatively. Where an electrical signal is used, it: has been found practical to use twoy switches, o ne to indicate a minimum platen spacing', on which signal pressure is reduced, and one to indicate a maximum spacing, on which signal pressure is increased. By this bleeding pressure in and out, the platens oscillate over a very small range of .006-inch per panel in a lO-.opening press. j

By the nature of the indication the operator with experience can determine trends in they variable characteristics of the material being pressed and thereby establish the corrective steps necessary to maintain specification density at the pressing` thickness.

Where the said two limit switches are employed they have been operated to light red and green signal lamps, to which the operator responds. Also, the signal circuits have been made to operate relays which control a reversible motor, arranged to increase and decrease the hydraulic pressure, thereby respectively to move the platens in closing or opening directions.

I claim-z l. The method which comprises forming a substantially uniformly thick and dense layer of a homogeneous mixture of comminuted woodV and hcat-activatable binder therefor, said mixture having a moisture content of at least 6 parts by weight per 100 parts of the mixture and having from 4 to 25 parts by weight of said binder to partsbone-dry weight of wood, placing said layer between spaced-apart heated platens having a temperature in the range from 250 F. to 400 F., promptly thereafter initiating movement to close said platens on said layer by pressure acting only against the resistance of said layer and thereby compressing said layer at a minimum spacing of the platens corresponding to a predetermined minimum thickness of an integrated panel to be formed from .sa-id mixture, said Aclosing of the platens on the layer being effected while the integrating action is incomplete. in substantially every part of thev layer, whereby thebinder. is thereafter fully activated'to integrate thel layer while the particles are substantially xedly positioned in said compressedlayer, and immediately after so-closing the platens regulating the said applied: pressure at lower/and lowering values substantially to maintain saidV minimumy spacing as back pressure from said: layer decreases tov a substantially constant value and said compressed layer is set substantially at said predetermined thickness, then opening the platens, and removing the resulting integrated panel.

2. The method which comprises forming a substantially uniformly thick and dense layer of a dry homogeneous mixture of comminutedv wood and a normally solid heat-activatable binder therefor, said mixturey havingy a moisture content of at least 6 parts by weight per 100 parts of the mixture and having from` 4 to 25. parts by weight of. said binder to- 1.00 parts bone-dryv weight of wood, placing said layer between spaced-apart heated platens having a temperature inthe range from ,250 F.. to 400 F., promptly thereafter'initiating movement to close said platens on said layer'by pressure 'acting only against the resistance of said layer and thereby compressing said layer at a minimum spacing of the platens corresponding to a predetermined minimum thickness of an integrated panel to be formed from said mixture, said closing of the platens on the layer being effected while the integrating action is substantially incomplete in every part of the layer, whereby the binder is lthereafter fully activated to integrate the layer while the particles are substantially xedly positioned in said compressed layer, and immediately after so closing the platens regulating the said applied pressure at lower and lowering values substantially to maintain said minimum spacing as back pressure from said layer decreases to a substantially constant value and said compressed layer is set substantially at said predetermined thickness, then opening the platens, and removing the resulting integrated panel.

3. The method which comprises forming a substantially uniform thick and dense layer of a homogeneous mixture of comminuted wood and a normally solid heatsoftening thermosetting binder therefor, said mixture having a moisture content of at least 6 parts by Weight per 100 parts of the mixture and having from 4 to 25 parts by weight of said binder to 100 parts bone-dry weight of wood, placing said layer between spaced-apart heated platens having a temperature in the range from 250 F. to 400 F., promptly thereafter initiating movement to close said platens on said layer by pressure acting only against the resistance of said layer and thereby compressing said layer at a minimum spacing of the platens corresponding to a predetermined minimum thickness of an integrated panel to be formed from said mixture, said closing of the platens on the layer being effected while the integrating thermosetting action is substantially incomplete in every part of the layer, whereby the binder is thereafter thermoset to integrate the layer while the particles are substantially xedly positioned in said compressed layer, and immediately after so closing the platens regulating the said applied pressure at lower and lowering values substantially to maintain said minimum spacing as back pressure from said layer decreases to a substantially constant value and said compressed layer is set substantially at said predetermined thickness, then opening the platens, and removing the resulting integrated panel.

4. The method which comprises forming a substantially uniformly thick and dense layer of a homegeneous mixture of comminuted wood and a normally solid heatsoftening thermosetting phenol-formaldehyde resin as binder therefor, said mixture having a moisture content of at least 6 parts by weight per 100 parts of the mixture and having from 4 to 25 parts by Weight of said binder to 100 parts bone-dry weight of wood, placing said layer between spaced-apart heated platens having a temperature in the range from 250 F. to 400 F., promptly thereafter initiating movement to close said platens on said layer by pressure acting only against the resistance of said layer and thereby compressing said layer at a minimum spacing of the platens corresponding to a predetermined minimum thickness of an integrated panel to be formed from said mixture, said closing of the platens on the layer being eieeted while the integrating thermosetting action is substantially incomplete in every part of the layer, whereby the binder is thereafter thermoset to integrate the layer While the particles are substantially xedly positioned in said compressed layer, and immediately after so closing the platens regulating the said applied pressure at lower and lowering values substantially to maintain said minimum spacing as back pressure from said layer decreases to a substantially constant value and said compressed layer is set substantially at said predetermined thickness, then opening tlhe platens, and removing the resulting integrated pane.

5. The method which comprises forming a substantially uniformly thick and dense layer of a dry homogeneous mixture of comminuted wood and a normally solid heat-softening thermosetting binder therefor, said mixture having a moisture content of at least 6 parts by weight per parts of the mixture and having from 4 to 25 parts by weight of said binder to 100 parts bonedry weight of wood, placing said layer between spacedapart heated platens having a temperature in the range from 250 F., to 400 F., promptly thereafter initiating closing movement of said platens on said layer by pressure acting only against the resistance of said layer and thereby compressing said layer at a minimum spacing of the platens corresponding to a predetermined minimum thickness of an integrated panel to be formed from said mixture, said closing of the platens on the layer being eifected while the integrating thermosettingaction is substantially incomplete in every part of the layer, whereby the binder is thereafter thermoset to integrate the layer while the particles are substantially xedly positioned in said compressed layer, and immediately after so closing the platens regulating the applied pressure at lower and lowering values substantially to maintain said mini- `mum spacing as back pressure from said layer decreases to a substantially constant value and said compressed layer is set substantially at said predetermined thickness, then opening the platens, and removing the resulting integrated panel.

6. The method which comprises forming a substantially uniformly thick and dense layer of a dry homogeneous mixture of comminuted wood and a normally solid heat-softening thermosetting phenol-formaldehyde resin as binder therefor, said mixture having a moisture content of at least 6 parts by weight per 100 parts ofl the mixture and having from 4 to 25 parts by weight of said binder to 100 parts bone-dry weight of wood, placing said layer between spaced-apart heated platens having a temperature in the range from 250 F. to 400 F., promptly thereafter initiating movement to close said platens on said layer by pressure acting only against the resistance of said layer and thereby compressing said layer at a minimum spacing of the platens corresponding to a predetermined minimum thickness of an integrated panel to be formed from said mixture, said closing of the platens on the layer being effected while the integrating thermosetting action is substantially incomplete in every part of the layer, whereby the binder is later thermoset to integrate the layer While the particles are substantially xedly positioned in said compressed layer, and immediately after so closing the platens regulating the applied pressure at lower and lowering values substantially to maintain said minimum spacing as back pressure from said layer decreases to a substantially constant value and said compressed layer is set substantially at said predetermined thickness, then opening the platens, and removing the resulting integrated panel.

References Cited in the le of this patent UNITED STATES PATENTS ame 

1. THE METHOD WHICH COMPRISES FORMING A SUBSTANTIALLY UNIFORMLY THICK AND DENSE LAYER OF A HOMOGENEOUS MIXTURE OF COMMINUTED WOOD AND HEAT-ACTIVATABLE BINDER THEREFOR, SAID MIXTURE HAVING A MOISTURE CONTENT OF AT LEAST 6 PARTS BY WEIGHT PER 100 PARTS OF THE MIXTURE AND HAVING FROM 4 TO 25 PARTS BY WEIGHT OF SAID BINDER TO 100 PARTS BONE-DRY WEIGHT OF WOOD, PLACING SAID LAYER BETWEEN SPACED-APART HEATED PLATENS HAVING A TEMPERATURE IN THE RANGE FROM 250* F. TO 400* F., PROMPTLY THEREAFTER INITIATING MOVEMENT TO CLOSE SAID PLATENS ON SAID LAYER BY PRESSURE ACTING ONLY AGAINST THE RESISTANCE OF SAID LAYER AND THEREBY COMPRESSING SAID LAYER AT A MINIMUM SPACING OF THE PLATENS CORRESPONDING TO A PREDETERMINED MINIMUM THICKNESS OF AN INTEGRATED PANEL TO BE FORMED FROM SAID MIXTURE, SAID CLOSING OF THE PLATENS ON THE LAYER BEING EFFECTED WHILE THE INTEGRATING ACTION IS INCOMPLETE IN SUBSTANTIALLY EVERY PART OF THE LAYER, WHEREBY THE BINDER IS THEREAFTER FULLY ACTIVATED TO INTEGRATE THE LAYER WHILE THE PARTICLES ARE SUBSTANTIALLY FIXEDLY POSITIONED IN SAID COMPRESSED LAYER, AND IMMEDIATELY AFTER SO CLOSING THE PLATENS REGULATING THE SAID APPLIED PRESSURE AT LOWER AND LOWERING VALUES SUBSTANTIALLY TO MAINTAIN SAID MINIMUM SPACING AS BACK PRESSURE FROM SAID LAYER DECREASES TO A SUBSTANTIALLY CONSTANT VALUE AND SAID COMPRESSED LAYER IS SET SUBSTANTIALLY AT SAID PREDETERMINED THICKNESS, THEN OPENING THE PLATENS, AND REMOVING THE RESULTING INTEGRATED PANEL. 